1. Field of the Invention
The invention relates to a lithographic projection apparatus, and more particularly to a lithographic projection apparatus that is compatible with a vacuum or semi-vacuum environment.
2. Discussion of Related Art
An apparatus of this type can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can then be imaged onto a target area (die) on a substrate (silicon wafer) which has been coated with a layer of photosensitive material (resist). In general, a single wafer will contain a whole network of adjacent dies which are successively irradiated through the reticle, one at a time. In one type of lithographic projection apparatus, each die is irradiated by exposing the entire reticle pattern onto the die in one go; such an apparatus is commonly referred to as a waferstepper. In an alternative apparatusxe2x80x94which is commonly referred to as a step-and-scan apparatusxe2x80x94each die is irradiated by progressively scanning the reticle pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the wafer table parallel or anti-parallel to this direction; since the projection system will have a magnification factor M (generally less than 1), the speed v at which the wafer table is scanned will be a factor M times that at which the reticle table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO 97/33205.
Up to very recently, apparatus of this type contained a single mask table and a single substrate table. However, machines are now becoming available in which there are at least two independently movable substrate tables; see, for example, the multi-stage apparatus described in International Patent Applications WO 98/28665 and WO 98/40791. The basic operating principle behind such a multi-stage apparatus is that, while a first substrate table is underneath the projection system so as to allow exposure of a first substrate located on that table, a second substrate table can run to a loading position, discharge an exposed substrate, pick up a new substrate, perform some initial alignment measurements on the new substrate, and then stand by to transfer this new substrate to the exposure position underneath the projection system as soon as exposure of the first substrate is completed, whence the cycle repeats itself; in this manner, it is possible to achieve a substantially increased machine throughput, which in turn improves the cost of ownership of the machine
In currently available lithographic devices, the employed radiation is generally ultra-violet (UV) light, which can be derived from an excimer laser or mercury lamp, for example; many such devices use UV light having a wavelength of 365 nm or 248 nm. However, the rapidly developing electronics industry continually demands lithographic devices which can achieve ever-higher resolutions, and this is forcing the industry toward even shorter-wavelength radiation, particularly UV light with a wavelength of 193 nm or 157 nm. Beyond this point there are several possible scenarios, including the use of extreme UV light (EUV: wavelengthxcx9c50 nm and less, e.g. 13.4 nm or 11 nm), X-rays, ion beams or electron beams. All of these so-called next-generation radiations undergo absorption in air, so that it becomes necessary to at least partially evacuate the environment in which they are employed. This introduces considerable problems.
A general discussion of the use of EUV in lithographic projection apparatus can be found, for example, in the article by J. B. Murphy et al. in Applied Optics 32 (24), pp 6920-6929 (1993). Similar discussions with regard to electron-beam lithography can be found in U.S. Pat. No. 5,079,112 and U.S. Pat. No. 5,260,151, as well as in EP-A 98201997.8 (P-0113.000-EP).
It is an object of the invention to provide a lithographic projection apparatus as stated in the opening paragraph, which apparatus is compatible for use in a vacuum or semi-vacuum environment. In particular, it is an object of the invention that such an apparatus should be compatible with the use of radiation comprising EUV, charged particles or X-rays. More specifically, it is an object of the invention that such an apparatus should not suffer from significant xe2x80x9cdown-timexe2x80x9d due to decrease of operational performance caused by degeneration of the projection system.
These and other objects are achieved in a lithographic projection apparatus that has a radiation system for supplying a projection beam of radiation; a mask table for holding a mask; a substrate table for holding a substrate; and a projection system for imaging an irradiated portion of the mask onto a target portion of the substrate. Preferably, the lithographic projection apparatus according to the invention has the following characteristics;
a) the projection system is separated from the substrate table by an intervening space which can be at least partially evacuated and which is delimited at the location of the projection system by a solid surface from which the employed radiation is directed toward the substrate table;
b) the intervening space contains a hollow tube located between the solid surface and the substrate table and situated around the path of the radiation, the form and size of the tube being such that radiation focused by the projection system onto the substrate table does not intercept a wall of the hollow tube;
c) means are provided for continually flushing the inside of the hollow tube with a flow of gas.
The xe2x80x9csolid surfacexe2x80x9d referred to under point (a) is, for example, the final mirror in the projection system from which the radiation is directed toward the substrate, or a (thin) optical flat (i.e. optical window) comprised of a vitreous material. The term vitreous should here be interpreted as encompassing such materials as silicates, quartz, various transparent oxides and fluorides (such as magnesium fluoride, for example) and other refractories.
In experiments leading to the invention, the inventors built a prototype device in which the radiation system delivered EUV (with a wavelength of approx. 13.4 nm). A projection system (comprising various mirrors) was used to focus the laser radiation onto a substrate table, onto which a test wafer could be mounted. A substantially evacuated enclosure, delimited (bounded) at one end by the exit aperture of the laser and at the other end by the substrate table, was provided around the projection system, so that the path of the radiation from source to substrate was substantially airless, including therefore the intervening space between the projection system and the substrate table. This intervening space was delimited on the side facing the substrate table by the last mirror in the projection system (the xe2x80x9csolid surfacexe2x80x9d referred to hereabove). Such evacuation was necessary because of the fact that EUV undergoes significant absorption in air, and was aimed at avoiding substantial light-loss at substrate level.
In working with this prototype system, the inventors observed rapid degeneration of the resolution and definition of fine (submicron-sized) images projected onto a resist-coated wafer on the substrate table. Many different possible sources of this problem were sought and investigated before the inventors finally observed that the final optical surface (mirror) in the projection system had become unacceptably contaminated. Further analysis demonstrated that this contamination was caused by the presence of a spurious coating of organic material, which was subsequently identified as consisting of debris and bye-products from the resist layer on the wafer. Evidently, such material was being xe2x80x9csputteredxe2x80x9d loose from the wafer by the EUV beam, and the evacuated intervening space between the wafer and the projection system allowed the released material to migrate toward the projection system (and other vicinal surfaces) without undergoings substantial scattering or deflection. Once arrived at the projection system, the material was adsorbed onto the highly accurate optical surfaces of the system, causing the said optical surface degradation.
In an effort to combat this problem, the inventors increased the distance between the substrate table and the projection system, but rapid contamination of the final optical surface of the projection system was still observed. Subsequent calculations (see Embodiment 1 below) revealed that such an approach was in fact doomed to be unsatisfactory, and that a more radical anti-contamination measure was required. Eventually, after trying various other approaches, the inventors arrived at the solution described in steps (b) and (c) above. In the inventive solution, the flush of gas prevents resist debris from reaching the projection system in the first place.
The gas employed in the flush should be a substance which does not substantially absorb the radiation in the projection beam (e.g. EUV), while having a substantially low diffusion coefficient for contaminants. An example of such a gas is Ar; an alternative is Kr, for example.
A particular embodiment of the apparatus according to the invention is characterized in that the hollow tube has the form of a cone which tapers inwards in a direction extending from the said solid surface towards the substrate table. Seeing as the projection system serves to focus an image onto the substrate, the radiation emerging from the projection system will taper inwards toward the final image on the wafer. If the employed hollow tube is of a conical form which imitates this said tapering, then the tube will have the minimal volume necessary to encapsulate the said emergent radiation. This is advantageous, since it minimizes the flow of gas required to produce an effective flush, leading to materials savings; in addition, the gas load to the system is reduced.
Another embodiment of the apparatus according to the invention is characterized in that the gas is introduced into the hollow tube via at least one opening in a wall of the tube. Alternatively, the gas can, for example, be introduced over a top rim of the tube. In a particular version of the former embodiment, the opening is a region which is porous to the employed gas.
Another embodiment of the apparatus according to the invention is characterized in that the flushing means are thus embodied that the flush of gas in the hollow tube is at least partially directed towards the substrate table. The very presence of gas at all (whether static or dynamic) between the substrate and the projection system provides a scattering barrier to debris migrating from the substrate. However, if such gas is additionally moved toward the substrate, then this provides an additional safeguard against such debris reaching the projection system. It should be noted that the flush need not be directed in its entirety towards the substrate: for example, if the gas is introduced via an opening in the wall of the tube located at some point (e.g. half way) between its upper and lower limits (rims), then some of the gas can flow from the hole upwards (toward the projection system) and the rest can flow downwards (toward the substrate).
In a manufacturing process using a lithographic projection apparatus according to the invention, a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of energy-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
Although specific reference has been made hereabove to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget areaxe2x80x9d, respectively.
Although the discussion in this text concentrates somewhat on the use of EUV, it should be explicitly noted that the invention is also applicable in systems employing other radiation types. For example, in the case of a lithographic apparatus employing UV light in combination with a (partially) evacuated environmentxe2x80x94aimed, for example, at reducing substrate contaminationxe2x80x94the current invention combats the built-up of resist debris on the UV projection optics. Similarly, in the case of electron or ion beam lithography, the present invention combats the build-up of substrate-produced contaminants on field-lens electrodes. In all cases, the present invention also combats the migration of debris from the substrate to the mask, radiation source, etc.